The present invention relates to a circuit for controlling the operation of a PNP transistor, and more particularly, to a circuit for reducing the delay in the transition time of a PNP transistor caused by stored base charge in the transistor as the transistor changes from a conducting state to a non-conducting state.
PNP transistors are often employed as switching elements in integrated circuits such as switching voltage regulators. Typically, during normal switching, the PNP transistor is cycled "on" and "off" to control a flow of current between the emitter and collector by intermittently supplying a forward biasing or drive current to the transistor. During "on" periods of the switch, an amount of charge is stored in the base of the transistor. When the drive current is removed, the emitter-base junction of the PNP transistor is caused to remain forwardbiased for a period of time by the stored charge, and the transistor continues to conduct current between its emitter and collector during the period of time required for the stored charge to decay. The stored charge increases the time for the transistor to transition from a conducting state to a non-conducting state, resulting in delayed switching. When the transistor is driven into saturation by the drive current, the charge stored in the base must be discharged before any reduction in the current conducted between the emitter and collector of the transistor can take place following the removal of the drive current. Because PNP transistors employed as switching elements often are driven into saturation to enable the transistors to conduct large currents efficiently, the delay in transition time caused by the stored charge may be several microseconds long, during which time a significant amount of current may be conducted by the PNP transistor after the drive current is removed from its base. This delay limits the switching speed of the transistor.
The discharge period of the stored charge can be reduced by applying a reverse drive current to the base of the switching PNP transistor. The greater the amount of reverse drive current which can be driven into the base of the PNP transistor, the faster the stored charge can be discharged. The resulting increase in the discharge rate of the stored charge reduces the transition time of the transistor. To drive the PNP transistor quickly into a non-conducting state, the reverse drive current must be able to drive the base voltage of the PNP transistor close to the level of the voltage at its emitter.
In view of the foregoing, it would be desirable to be able to provide a circuit for generating a reverse drive current which is capable of driving the base voltage of a PNP transistor substantially close to the level of the voltage at its emitter to discharge the stored charge in the base of the PNP transistor.